1. Field of the Invention
The present invention relates to a device for controlling hydraulic pressure for braking a vehicle and more particularly to an improved device for controlling hydraulic pressure for braking a vehicle of the type including a pressure reducing valve disposed midway of an oil passage extending between an output port of a master cylinder and a rear wheel brake, said pressure reducing valve being adapted to proportionally reduce hydraulic pressure fed from the output port and then transmit reduced hydraulic pressure to the rear wheel brake, so that an occurance of locking of rear wheels is inhibited, said locking being attributable to decrease of load exerted on the rear wheels when braking is effected.
2. Description of the Prior Art
When a vehicle is braked with the aid of a device of the above-mentioned type, there is necessity for changing hydraulic pressure at which a pressure reducing valve initiates pressure reducing function in dependence on the weight of cargoes loaded on the vehicle, for instance, determining hydraulic pressure at which the pressure reducing valve initiates its pressure reducing function when the vehicle is loaded with cargoes to be higher than that at a time when the vehicle is not loaded.
Since the conventional device is constructed so that hydraulic pressure at which the pressure reducing valve initiates pressure reducing function is determined by preset load of spring means, the aforesaid hydraulic pressure is always kept constant and therefore no possibility exists of changing such pressure in dependence on the weight of cargoes loaded on the vehicle.
Hence, the present invention has been made with the foregoing problems in mind and it is an object of the invention to provide an improved device for controlling hydraulic pressure for braking a vehicle of the type adapted to change hydraulic pressure at which a pressure reducing valve initiates pressure reducing function with the aid of a pneumatic spring as employed for a suspension system dependently on the weight of cargoes loaded on the vehicle.
It is other object of the invention to provide an improved device for controlling hydraulic pressure for braking a vehicle of the type which assures that the pressure reducing valve carries out pressure reduction with high operational safety even when a pneumatic system including a pneumatic spring fails to function properly.
To accomplish the above objects there is proposed in accordance with the present invention an improved device for controlling hydraulic pressure for braking a vehicle of the type including a pressure reducing valve disposed midway of an oil passage extending between an output port of a master cylinder and a rear wheel brake, said pressure reducing valve being adapted to proportionally reduce hydraulic pressure fed from said output port and then transmit reduced hydraulic pressure to the rear wheel brake, the improvement consisting in that a pneumatic spring incorporated in a suspension mechanism for determining hydraulic pressure at which the pressure reducing valve initiates pressure reducing function dependently on the weight of cargoes loaded on the vehicle is operatively connected to a valve body of the pressure reducing valve in such a manner as to bias said valve body in a valve opening direction.
In a preferred embodiment of the invention the device includes a mechanical spring which serves to determine hydraulic pressure at which the pressure reducing valve initiates pressure reducing function when the pneumatic spring fails to function properly, said mechanical spring being in operative connection to the valve body of the pressure reducing valve in such a manner as to displace it in the direction of valve opening.
In another preferred embodiment of the invention it is proposed that the pneumatic spring comprises a first pneumatic spring and a second pneumatic spring and the pressure reducing valve includes a cylinder additionally mounted thereon into which a free piston is slidably fitted to define a first pressure chamber and a second pressure chamber therein, said first and second pressure chambers being connected to the valve body of the pressure reducing valve as well as being communicated with said first and second pneumatic springs, respectively.
The arrangement of the device in the above-described manner makes it possible to appropriately determine hydraulic pressure at which the pressure reducing valve initiates its pressure reducing function in dependence on the weight of cargoes loaded on vehicle and therefore braking can be effected similarly to ideal braking.
In the first mentioned embodiment where the device includes a mechanical spring disposed therein, the spring determines hydraulic pressure at which the pressure reducing valve initiates its pressure reducing function when a pneumatic system including the pneumatic spring falls into trouble for some reason, thereby to reduce hydraulic pressure to be transmitted to the rear wheel brake. Thus, it is assured that occurance of locking of rear wheels is prevented, resulting in high operational safety.
Further, in the last mentioned embodiment the pressure reducing valve includes a cylinder additionally mounted thereon into which a piston is slidably fitted to define a first pressure chamber and a second pressure chamber therein, said first and second pressure chambers being connected to the valve body of the pressure reducing valve as well as being communicated with first and second pneumatic springs in a suspension system, respectively, which springs serve to displace the valve body in the valve opening direction thereby to determine hydraulic pressure at which the pressure reducing valve initiates its pressure reducing function dependently on the weight of cargoes loaded on vehicle, whereby pressure reduction is effected with the aid of either one of pneumatic systems including one pneumatic spring when the other system is in trouble. Thus, the same advantages as in the first mentioned embodiment are assured.
Above and other objects, features and advantages of the present invention will become more clearly apparent from reading of the following detailed description of some preferred embodiments in conjunction with the accompanying drawings.